"What do you think of solid piles as against hollow ones?"
"Well, I heard a discussion between two engineers about it, and they agreed that solid piles only do for little or medium heights, and I asked one to write a line or two for my guidance, and this is what he dashed off. Read it."
"No. Read it to me."
"Well, it runs:—'In designing solid piles it should be remembered that the strength of solid round columns to resist torsion, torsional strength (he means strength against twisting strain) is as the cubes of their diameters, therefore a solid round bar 4 inches in diameter will bear eight times the torsional strain of a bar 2 inches, the lengths being the same.'"
"How's that?"
"Why, 2 × 2 × 2 = 8, 4 × 4 × 4 = 64, and 64/8 = 8."
"I understand."
"In the case of hollow columns, the exterior diameter must be cubed, and the cube of the interior diameter deducted from it when the relative values of different-sized columns can be compared. For transmitting motion, and here torsional stiffness is referred to, the resistance of shafts of equal stiffness is proportional to the fourth power of their diameters. A 2-inch shaft will transmit 16 times the force which would be transmitted by a 1-inch shaft without being twisted through a greater angle. When the height of the pile is considerable the diameter should be relatively larger, in order that the metal may not be subject to severe torsional strain. So don't forget the piles should be of large and not small diameter, or you may have trouble in screwing them."
"You remember old Bill Marr?"
"Rather, who did the iron pilework on the Shore Railway. I should think I did, for old Spoil'em, we called him, and I were in 'Co.' together more than once."